Gutter Sizing


Old 06-06-13, 03:05 PM
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Gutter Sizing

The one area of my roof is 1242 sq feet
The pitch is 6 in 12
Rainfall factor for my region is 7.1
That makes my adjusted square footage = 9700 sq ft
Current gutters are 5 in K Style with two 2x3 downspouts

I want to replace the gutters and down spouts. All roofing and siding contractors in my area use only 5 and 6 inch K Style gutters and none of them know anything about how to determine the right size gutters. I read somewhere that the max square footage for a 6 inch K Style gutter is 7960 sq ft. Is that correct? Will 6 inch K Style gutters be large enough to handle the drainage for a roof this size? Thanks
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Old 06-06-13, 03:32 PM
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6" will in all probability carry the water. Your problem is the downspouts. If you are gathering 6" of water (at whatever volume), and try to push it through a 2x3 downspout, it will back up. Increase the downspout size accordingly (5").
Old 06-06-13, 04:02 PM
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Nothing is better than having a gutter, and especially downspouts over sized than minimal.

Our townhouse (4 - 2 1500sf story units per rectangle) association got rid of the junk and ineffective "gutter guards" and went to larger seamless gutters and downspouts that were larger. That was two or three years ago. Now, with the bigger gutters and downspouts (without the stupid gutter guards) all problems with leaks from the surrounding large trees we have had a marked improvement in fast drainage and elimination of ice dams since everything is free flowing. - Despite near record snow and low temps in the winter and plenty of rain here.

Old 06-08-13, 07:10 AM
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More importantly is the location and number of downspouts. Adding a 2 by 3 rectangular downspout, boosts your gutter's capacity by 600 square feet of drainage area. One downspout on a 40 ft run of gutter with proper slope 1/4" / 10ft will work most of the time. Add downspouts as needed, 2 by 3 = 600 square feet, 3 by 4 = 1,200 square feet, Increase the slope of the gutter will help too.
Old 06-09-13, 07:08 AM
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Thanks for the replies. It sounds like 6 inch gutters will work. The section of roof I referred to in my original post is 46 feet long with 2 downspouts (2x3). So if I install 6 inch gutters and 3x4 inch downspouts, that should have sufficient capacity to handle all the water.

7960 + 1200 + 1200 = 10360

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